A rocket body bears huge takeoff thrust in the launching phase, and thus the axial compressive load is the most important consideration to design a load-carrying structure of the body. A propellent tank reinforced by stiffeners bears huge axial compression even though the propellent tank is a secondary load-carrying structure of the rocket body. A new generation of large-diameter launch vehicle named CZ-5 is developed in China, and the diameter of the core structure is 5 m. Take the CZ-5 for example, even a liquid oxygen tank reinforced by stiffeners and having a diameter of 3.35 m in the booster system bears an axial compression load over 4000 kN. However, an axially compressed thin-walled component is very sensitive to initial imperfections, especially to initial geometrical imperfections, causing the ultimate load-carrying capacity of the structures estimated on the basis of a perfect model theory or data to be much smaller than that of the actual condition. Engineers always employ a “knockdown factor” (or correction factor) which is much smaller than 1 to correct the estimated load-carrying capacity. In general, when the ratio of radius to equivalent skin thickness of the shell is larger, imperfection sensitivity is larger, the knockdown factor is smaller, and an allowable load-carrying capacity employed in structure design is smaller when compared with the load-carrying capacity estimated based on the perfect model. As the launching load of the new generation of launch vehicles and heavy-lift launch vehicles in the future improve by leaps and bounds, the diameters of the rockets also tend to largely increase, and the imperfection sensitivity of load-carrying cylindrical shells becomes increasingly prominent, thus it is crucial to develop a novel method for determining the knockdown factor of load-carrying capacity of the cylindrical shell under axial compression.
Conventional methods for evaluating imperfection sensitivity of cylindrical shell, represented by NASA SP-8007, mainly employ a semi-empirical formula and yield the knockdown factor of the load-carrying capacity of the cylindrical shell based on a large quantity of experiments. With the development of manufacturing technology and material system, the conventional methods turn out to be extremely conservative, bringing in much costs and design redundancy. In view of the above-mentioned problems, many specialists employ the numerical analysis method to investigate the imperfection sensitivity of cylindrical shells. According to the method, initial imperfections such as an imperfection with first order eigen-mode shape, imperfection caused by radial perturbation load, and single dimple imperfection were introduced to the perfect cylindrical shell, and then the knockdown factor of the load-carrying capacity of the structure was yielded after calculating the data. Though large amount of related work has been carried out, a more physical method for determining the knockdown factor in consideration of the realistic worst imperfections has not been provided.
In conclusion, it is necessary to put forward an improved method for determining the knockdown factor of the load-carrying capacity of the cylindrical shell, which is convenient to be verified via experiments.